Following the evolution path of social commercialization and industrialization, increasing indoor space is utilized and more carriers are used for satisfying people's needs in rest, work, and commute. Nonetheless, when people are situated in closed indoor space, accumulation of hazardous gas will occur due to air noncirculation. At best, the living quality of the people in the space will be affected. At worst, it may hurt human bodies directly. In general, when the indoor concentration of carbon dioxide is below 1,000 ppm, it is considered normal and the air circulation is good. As the indoor concentration of carbon dioxide is raised to 1,000 ppm˜2,000 ppm, oxygen is insufficient. People may feel tired and become anxious. When the indoor concentration of carbon dioxide is further increased above 5,000 ppm, anoxia may occur, leading to permanent brain injury, coma, or even death. In actual daily measurement, owing to insufficient air exchange of the indoor air conditioner or excessive number of people in the space, the actual measurement of carbon dioxide concentration can reach around 2,000 ppm˜3,000 ppm, which is a condition making people feel drowsy or slight uncomfortable. In a further action for controlling the indoor concentration of carbon dioxide is not performed, it may continue to rise. Then the people in the space are exposed to danger.
On the other hand, carbon monoxide is the gas requiring more monitoring and control in our daily lives. Carbon monoxide is a colorless and smell-less chemical formed by incomplete combustion of carbon-containing substances. Thereby, the incomplete combustion of fuel gas or motorcycle exhaust makes people to be exposed to carbon monoxide in the living environment. Because the affinity of hemoglobins with carbon monoxide is higher than with oxygen by two to three hundred times, when a human inhales carbon monoxide, it will compete with the oxygen in the body for the possibility of combining with hemoglobins. It replaces oxygen and combines with hemoglobins. The oxygen level in human blood decreases. People will lose consciousness, become stuporous gradually without notice and eventually die owing to heart and brain damages. Given the threat on lives of carbon monoxide, early detection of increases in carbon monoxide in closed space becomes a crucial subject.
The current commercial gas sensors are mainly infrared gas sensors. The infrared supplies energy for exciting gas and producing changes in temperature, displacement, or frequency. According to the absorption of infrared by gas and detecting the characteristic absorption peaks, the type and concentration of gas can be judged. Although the accuracy sensing gas using infrared is high, it is sensitive to ambient temperature. Besides, due to its large size, high cost, and difficulty in miniaturization, the promotion of infrared gas sensors is challenging.
In addition, there is another gas sensor that detects gas by using semiconductors. Metal-oxide materials are sintered to form semiconductors. By keeping the heater at a high temperature, semiconductor metal oxides contact an inflammable gas. Once the resistance is related to the gas concentration, the detection for carbon monoxide can be achieved. Although the device is simple for monitoring, the circuitry is vulnerable to temperature and humidity. Besides, the accuracy of the sensor is also vulnerable to the thermoelectric effect of semiconductors.
Based on the above description, it is understood that the detection of gas concentration is closely related to the safety of indoor space. Unfortunately, current commercial gas sensors are limited in functions. Accordingly, how to provide a miniature and accurate gas sensor has become the major technical challenge in the field.